1. Field of the Invention
This invention is directed to a method of forming a glass melt, and in particular forming a glass melt utilizing a multi-zone melter.
2. Technical Background
Liquid crystal displays (LCDs) are passive flat panel displays which depend upon external sources of light for illumination. Alkali-free alumino-silicate glasses are commonly used for LCD sheet glass applications. This family of glasses tends to create a stable foam layer on the surface of the melting furnace (melter) in a location where the batch (glass feed) materials are filled. The foam layer contains solid silica inclusions that can become either solid “stone” or clear “knot” defects in the finished glass unless they are removed prior to the glass entering the delivery system. It has been shown that this foam layer, when allowed to reach the front wall of the melter, can deliver solid inclusions via the melter exit to the glass delivery system. These solid inclusions can become solid defects in the finished glass. The foam layer also insulates the glass melt from heat supplied by combustion burners above the free surface of the glass melt. The subsequent poor efficiency of the burners means that most of the energy required to form the melt is provided by Joule heating from electrodes submerged below the free surface of the melt. The resulting relatively high levels of electric power shortens electrode life and leads to frequent melter repairs.
A single melter with two or more zones can prevent the silica inclusions resident in the foam layer from entering the glass delivery system. A wall separating the first and second zones can prevent the foam layer in the first zone from entering the second zone. Historically, division of a melter into multiple zones has been done with either an internally-cooled cross wall with one or more slot-shaped throats (dividing one large glass bath into two smaller zones) or with two separate chambers connected with a tunnel-shaped throat.
In the case of the cross wall, both sides of the cross wall are hot and generally corrosion of the wall by the glass is relatively fast. Thus, process life is short. Melting effectiveness ends when the top of the cross wall is breached or when internal cooling fails, releasing cooling water directly (and explosively) into the glass melt. Furthermore, if the cross wall is constructed of fused zirconia refractory, the electrical resistivity of the cross wall will be low, and both faces will be hot. Some of the electric current used to heat the glass bath may pass through the cross wall, heating it independently and potentially causing failure of the wall or formation of zirconia inclusions in the melt. Generally, cross walls are effective for limited periods of time but represent a life-limiting part of a glass melting process.
The conventional approach to these problems is to enlarge the melter. It is estimated that to achieve a foam-free surface would require at least a doubling of the present-day melt surface area. Further, to reduce solid and gaseous inclusions to the desired level would require another multiple, bringing the total enlarged melter size to three times the present day surface area. Such large increases in the dimensions of the melting furnace lead to increased capital and operating expenses, and, because the number of electrodes (typically tin oxide) would necessarily increase, may also result in raising the amount of tin oxide in the glass to levels where Cassiterite devitrification of the melt can occur.
Melters can also be separated into zones that do not share a common wall. In this case, the first and second zones may have their own walls that are connected by a tunnel-shaped throat. This allows the walls to have exterior cooling but creates a significant unheated area within the melter where the glass can lose temperature as it passes from the first zone to the second zone. The effectiveness of the second zone in melting out solid inclusions or fining out gaseous inclusions diminishes when the glass enters the second zone colder than it exited the first zone. In addition, refractory throat covers will wear away to the glass level, ultimately allowing the foam layer to pass through from the first to the second zone. A throat leak can cause shutdown of the process altogether.
For a two-zone melter to be effective in keeping the solid inclusions entrained within the foam layer from entering the delivery system, the separation between the first and second zones must retain its integrity. Otherwise, the melter becomes one large container that allows the foam layer to move forward to the front-wall and deliver solid inclusions from the foam layer into the glass delivery system.
When a melting process comprised of two or more zones is effective, the foam layer is prevented from forming in the second zone and additional time and temperature is available in the second zone to melt out the solid inclusions or fine out gaseous inclusions that enter it.